In a synchronous communication system that synchronizes a transmission-side unit and a reception-side unit, the transmission-side unit transmits a pilot signal (reference signal (RS)) from each transmission antenna and the reception-side unit carries out a process of estimating a propagation path based on the RS. In recent years, a method of using multiple antennas, such as Multiple Input Multiple Output (MIMO), has been proposed for radio communication. According to this method, an RS is transmitted from each transmission antenna to acquire a propagation path for each antenna. In this case, when an RS is transmitted from a given antenna, other antennas are not allowed to transmit signals. As the number of transmission antennas increases, therefore, overhead for transmission of RSs other than data increases, which leads to a decline in transmission efficiency.
To reduce the volume of RS transmissions, a method has been conceived, according to which some antennas among multiple antennas transmit RSs. For example, when transmission-side antennas are close to each other over an extremely small interval, the correlation between propagation paths is large. For this reason, the transmission-side unit transmits RSs from some antennas and the reception-side unit estimates propagation paths for antennas other than the antennas having transmitted the RSs by an interpolating process. This method is known as a method for improving data transmission efficiency (see, e.g., Japanese Laid-Open Patent Publication No. 2007-089113).
When the transmission-side unit is a small-sized mobile terminal, antennas are installed in a limited space and the interval between the antennas tends to be narrower. In such a case, the above method of culling RSs is considered to be effective. However, for some mobile terminals, a variation in the shape of the terminal results in a variation in the intervals between antennas. For example, in a case of a foldable mobile terminal or slidable mobile terminal, the distance between antennas when the terminal is closed is different from that when the terminal is opened. Consequently, the correlation between the antennas varies depending on whether the terminal is opened or closed. For example, opening the mobile terminal widens the antenna interval, thus reducing the correlation, while closing the terminal narrows the antenna interval, thus increasing the correlation. For this reason, in the case of a mobile terminal whose shape varies, if the number of RS transmissions is fixed at the transmission-side unit, the fixed number of RS transmissions is not the optimal number. Cellular phones, PDA, portable personal computer, etc., are examples of as such mobile terminals.
In the conventional communication system, when the correlation between antennas of the mobile terminal varies, the transmission-side unit cannot perform transmission by the optimal number of RS transmissions corresponding to the correlation variation nor suppress the transmission of unnecessary RSs from unselected antennas. The transmission of unnecessary RSs makes reductions in the transmission power impossible and raises a concern of interference with other reception-side units (base station) or other mobile terminals. When the transmission-side unit is a mobile terminal, a variation in the shape causes the correlation between antennas to vary. The mobile terminal is not able to deal with this correlation variation and is, therefore, not able to transmit/receive main data using the optimal number of RS transmissions. The reception-side unit not knowing the optimal number of RS transmissions is incapable of carrying out accurate antenna interpolation. Hence, transmission efficiency and quality cannot be improved.